Developments have resulted in revision of the taxonomy of the Thraustochytrids. Taxonomic theorists place Thraustochytrids with the algae or algae-like protists. However, because of taxonomic uncertainty, it would be best for the purposes of the present invention to consider the strains described in the present invention as Thraustochytrids (Order: Thraustochytriales; Family: Thraustochytriaceae; Genus: Thraustochytrium, Schizochytrium, Labyrinthuloides, or Japonochytrium). Taxonomic changes are summarized below.
Strains of certain unicellular microorganisms disclosed and claimed herein are members of the order Thraustochytriales. Thraustochytrids are marine eukaryotes with a problematic taxonomic history. Problems with the taxonomic placement of the Thraustochytrids have been reviewed by Moss (1986), Bahnweb and Jackle (1986) and Chamberlain and Moss (1988).
For convenience purposes, the Thraustochytrids were first placed by taxonomists with other colorless zoosporic eukaryotes in the Phycomycetes (algae-like fungi). The name Phycomycetes, however, was eventually dropped from taxonomic status, and the Thraustochytrids were retained in the Oomycetes (the biflagellate zoosporic fungi). It was initially assumed that the Oomycetes were related to the heterokont algae, and eventually a wide range of ultrastructural and biochemical studies, summarized by Barr (1983) supported this assumption. The Oomycetes were in fact accepted by Leedale (1974) and other phycologists as part of the heterokont algae. However, as a matter of convenience resulting from their heterotrophic nature, the Oomycetes and Thraustochytrids have been largely studied by mycologists (scientists who study fungi) rather than phycologists (scientists who study algae).
From another taxonomic perspective, evolutionary biologists have developed two general schools of thought as to how eukaryotes evolved. One theory proposes an exogenous origin of membrane-bound organelles through a series of endosymbioses (Margulis 1970); e.g., mitochondria were derived from bacterial endosymbionts, chloroplasts from cyanophytes, and flagella from spirochaetes. The other theory suggests a gradual evolution of the membrane-bound organelles from the non-membrane-bounded systems of the prokaryote ancestor via an autogenous process (Cavalier-Smith 1975). Both groups of evolutionary biologists however, have removed the Oomycetes and Thraustochytrids from the fungi and place them either with the chromophyte algae in the kingdom Chromophyta (Cavalier-Smith 1981) (this kingdom has been more recently expanded to include other protists and members of this kingdom are now called Stramenopiles) or with all algae in the kingdom Protoctista (Margulis and Sagan (1985).
With the development of electron microscopy, studies on the ultrastructure of the zoospores of two genera of Thraustochytrids, Thraustochytrium and Schizochytrium, (Perkins 1976; Kazama 1980; Barr 1981) have provided good evidence that the Thraustochytriaceae are only distantly related to the Oomycetes. Additionally, genetic data representing a correspondence analysis (a form of multivariate statistics) of 5 S ribosomal RNA sequences indicate that Thraustochytriales are clearly a unique group of eukaryotes, completely separate from the fungi, and most closely related to the red and brown algae, and to members of the Oomycetes (Mannella et al. 1987). Most taxonomists have agreed to remove the Thraustochytrids from the Oomycetes (Bartnicki-Garcia 1988).
In summary, employing the taxonomic system of Cavalier-Smith (1981, 1983), the Thraustochytrids are classified with the chromophyte algae in the kingdom Chromophyta, (Stramenopiles). This places them in a completely different kingdom from the fungi, which are all placed in the kingdom Eufungi. The taxonomic placement of the Thraustochytrids is therefore summarized below:    Kingdom: Chromophyta (Stramenopiles)    Phylum: Heterokonta    Order: Thraustochytriales    Family: Thraustochytriaceae    Genus: Thraustochytrium, Schizochytrium, Labyrinthuloides, or Japonochytrium 
Some early taxonomists separated a few original members of the genus Thraustochytrium (those with an amoeboid life stage) into a separate genus called Ulkenia. However it is now known that most, if not all, Thraustochytrids (including Thraustochytrium and Schizochytrium), exhibit amoeboid stages and as such, Ulkenia is not considered by some to be a valid genus. As used herein, the genus Thraustochytrium will include Ulkenia. 
Despite the uncertainty of taxonomic placement within higher classifications of Phylum and Kingdom, the Thraustochytrids remain a distinctive and characteristic grouping whose members remain classifiable within the order Thraustochytriales.
Schizochytrium and other Thraustochytriales microorganisms have substantial existing and potential commercial value because of their ability to produce large quantities of lipoidal compounds, including highly unsaturated fatty acids (HUFAs) and various carotenoids (e.g., astaxanthin). Omega-3 highly unsaturated fatty acids are of significant commercial interest in that they have been recently recognized as important dietary compounds for preventing arteriosclerosis and coronary heart disease, for alleviating inflammatory conditions and for retarding the growth of tumor cells. These beneficial effects are a result both of omega-3 HUFAs causing competitive inhibition of compounds produced from omega-6 fatty acids, and from beneficial compounds produced directly from the omega-3 HUFAs themselves (Simopoulos et al., 1986). Omega-6 fatty acids are the predominant HUFAs found in plants and animals. Therefore, further development of Thraustochytriales microorganisms as commercial production organisms will benefit significantly from the ability to make specific genetic changes to the organisms via recombinant DNA technology, including enhancing the production of the highly valuable HUFAs and carotenoids by such organisms. In addition, the ability to gain a better understanding of the biochemistry and molecular biology of this poorly characterized group of organisms will provide valuable information that can be used to guide future strain development efforts. Prior to the present invention, however, methods and recombinant constructs suitable for transforming Thraustochytrids, including members of the genera, Schizochytrium and Thraustochytrium were not available. Importantly, the development of selectable markers that are particularly useful for transforming Thraustochytriales microorganisms and the identification of Thraustochytriales-specific promoter sequences were not available prior to the present invention.
Prior investigators have described transformation methods and reagents for use in various microorganisms, including in microalgae that are not members of the Thraustochytriales order. U.S. Pat. No. 6,027,900 to Allnutt et al. discloses genetic fusions for use in genetic engineering of eukaryotic algae, and particularly, Phaeodactylum tricornutum, using a promoter for a photosynthetic algal light harvesting gene and the Sh ble gene from Streptoalloteichus hindustanus as a selectable marker. The cells are grown in high concentrations of salt (e.g., 10-35 g/L) and Zeocin™ for selection of transformants. The microalgal cells suitable for transformation using such a method are photosynthetic microalgae that can be grown under the high salt conditions. U.S. Pat. No. 5,661,017 to Dunahay et al. discloses a method to transform cholorophyll C-containing algae (e.g., Diatoms) using a recombinant construct comprising a selectable marker operatively linked to a regulatory control sequence suitable for expression of the marker in the cholorophyll C-containing algae. The selectable marker is disclosed as being any suitable marker, including markers isolated from bacterial and fungal sources, and is preferably neomycin phosphotransferase. The regulatory control sequence can include any regulatory sequence derived from a cholorophyll C-containing algae, and preferably, from Cyclotella cryptica (e.g., a C. cryptica acetyl-CoA carboxylase regulatory sequence).
However, such methods are not readily transferable to the transformation of Thraustochytriales microorganisms, because, prior to the present invention, the transformation of microorganisms such as Thraustochytriales (e.g., microalgae) was far from routine. Markers and transformation systems that have become well developed for bacteria and yeast are not necessarily readily adaptable to other microorganisms. Indeed, U.S. Pat. No. 5,661,017 notes that “there has been little success in developing transformation systems for eucaryotic microalgae” (col. 1, lines 49-51), which is partly due to the difficulty of introducing foreign DNA into such microorganisms, and partly due to a lack of suitable markers and vectors for use in such transformation. The system described in U.S. Pat. No. 5,661,017 was developed specifically for the chlorophyll C-containing algae because those inventors believed them to be amenable to genetic transformation, particularly as compared to other algae. Similarly, U.S. Pat. No. 6,027,900, which teaches a transformation method that is specific to photosynthetic microalgae, speaks to the belief that most algae are refractory to any type of genetic manipulation (col. 1, lines 39-47). The systems adapted for bacteria, yeast, insect and animal cells have not been readily adapted to microalgae. Therefore, prior to the present invention, there was still a need in the art for effective transformation systems and methods that are specific for microalgae.
Additionally, although the order Thraustochytriales is now grouped with the chromophyte algae in the Stramenopiles, there is still an opinion by some in the art that these microorganisms are quite different from most microalgae, and some of those of skilled in the art have the opinion that Thraustochytriales members may not be properly classified as microalgae at all. Microorganisms considered to be microalgae have evolved at least four separate times during evolution, leading the “microalgal” type microorganisms to be placed in different kingdoms (e.g. the red algae, green algae and golden algae (Chromophyta) are all in separate kingdoms). As a result, transformation systems that have been demonstrated to be useful in other microalgae are not expected to be useful for Thraustochytriales. Therefore, despite the commercial value of Thraustochytriales microorganisms, the ability to make use of the full potential of such microorganisms by genetic engineering has not heretofore been realized. Prior to the present invention, the present inventors were not aware of any promoters, selectable markers, or vectors useful for transformation of Thraustochytriales microorganisms, nor was there any knowledge regarding what selection systems could be used in or adapted to Thraustochytriales.
In summary, there is a need in the art to develop methods for transforming Thraustochytriales microorganisms, thereby providing a means to create strains with enhanced commercial value. In addition, there is a need in the art to develop methods for mutation or inactivation of specific genes by homologous or nonhomologous recombination in Thraustochytriales microorganisms, providing a new way to alter cellular metabolism and to identify the functions of specific genes in Thraustochytriales.